Power converters are useful in many applications, such as converting Alternating-Current (AC) from a power outlet to Direct Current (DC) for use in electronic devices. In cathode-ray tubes (CRT's), a fly-back inverter was used to block the electron beam when the raster dot was retracing or flying back to the beginning on the next raster line in the display. This class of power converters is still known as a fly-back converter, even though they are used in many non-CRT applications, such as for driving light-emitting diodes (LED's) and chargers.
Control circuitry is needed to produce a constant DC output current. A transformer may be used to isolate the primary side connected to the AC power from the DC output side for enhanced safety. Opto-isolators have been used to isolate the control circuitry, but these opto-isolators are relatively expensive and do not integrate well with integrated circuits (IC's).
Rather than connect the control circuitry to the secondary (DC) side of the transformer, the control circuitry can connect to the primary (AC) side. Heat dissipation, form factors, component counts, and costs can be reduced with primary-side sensing regulators (PSR) and control due to the higher efficiency and elimination of the opto-isolator.
While a variety of fly-back converters are known, many of these only support Pulse-Width-Modulation (PWM) control. Some applications could benefit from other control methods, such as Quasi-Resonant (QR), which yields reduced Electro-Magnetic Interference (EMI) and is better suited for solid-state lighting and charger applications.
What is desired is a fly-back converter with primary-side control that does not need an opto-isolator. A fly-back converter that supports both PWM and QR control is desirable that produces a constant current drive. Current accuracy is desirable using a circuit that is modeled by simple equations.